1. Field of the Invention
This invention relates to lens mountings and more particularly to lens mountings having incorporated therein a motor in the hollow cylindrical form of a field coil and a field magnet.
2. Description of the Prior Art
Many proposals have been made for using an electromagnetic induction mechanism to move a member or unit within the interior of a lens barrel.
For example, U.S. Pat. Nos. 3,687,042 and 4,113,359 disclose a diaphragm unit in a lens mounting operated by an electromagnetic induction mechanism.
Also, a small-sized D.C. motor has been incorporated in a lens mounting of cine cameras to serve as the motorized operating mechanism for the focusing lens. The drive torque of this motor is transmitted through a gear train to the lens holder so that the lens moves axially.
Prior art motorized focusing mechanisms have the problem that the necessity of putting the motor on the outer barrel of the lens mounting makes the outer appearance of the housing of the lens mounting awkward. Common usage of the term "lens barrel" as meaning integrally cylindrical gives the above-described prior art mechanisms the impression of incompatability so they are not preferred industrial designs. Another problem is that the increased complexity of prior art mechanical mountings results in design and manufacturing disadvantages.
It is also known to apply the principles of linear motors to control axial movement of the focusing lens, as disclosed in U.S. patent application Ser. No. 396,030 filed July 7, 1982 (corresponding to Japanese Laid-Open Patent Application No. SHO 58-10706 published Jan. 21, 1983). In this linear motor lens drive mechanism, the axial length of the stator of the motor must be elongated depending upon the range of movement of the focusing lens. Further, fine position adjustment of the focusing lens is also difficult to achieve.
Another type of focusing mechanism for an objective lens is disclosed in U.S. Pat. No. 4,152,060. This patent discloses an epicyclic motor which has a stator concentric to the optical axis and a tube shape armature eccentric to the optical axis. Upon energization, the armature epicyclically rotates relative to the stator. A lens holder having a central axis substantially coincident with the optical axis of the lens contained in the holder is rotatably mounted in the interior of the tubular armature through an intermediate sleeve. The center of rotation of this sleeve is concentric to the optical axis. When the intermediate sleeve rotates, the lens holder is driven axially. This patent thus discloses a focusing mechanism with a drive connection between the armature and the lens holder to transmit epicyclic motion of the armature to rotate the lens holder. Because of this complicated structure, the assembling and adjusting operations of the lens mounting production line are very difficult to carry out.
To mount a lens of circular lateral cross-section, the lens holder, the stationary body, the actuator for the focusing component and/or zoom component, the helicoid member, the cam member, and all other constituent parts should have a circular lateral cross-section. For this reason, the artisan has been seeking a technique that allows the drive source for the focusing or zoom component to be constructed with parts of tubular form compatible with the constituent parts of the lens mounting. This would provide an elegant outer appearance without any awkward protuberance.
Such a technique has been disclosed in Japanese Laid-Open Patent Application No. SHO 57-186738 published Nov. 17, 1982. This prior known lens mounting has a lens holder sleeve including a group of lens elements arranged to be moved axially as a unit. An air space is provided near the lens holder. A motor is positioned in this air space and fixedly secured to the lens holder. When the motor is energized, the holder sleeve with the group of lens elements is driven to move axially as a unit with the motor to effect either focusing or zooming of the photographic object. In one illustrated embodiment of the prior known invention, the motor for driving the lens has a hollow cylindrical form.